1. Field of the Invention
The present invention relates to a membrane-electrode assembly for a solid polymer electrolyte fuel cell.
2. Related Art
In general, electrolytes are often used in solutions, typically aqueous solutions. However, in recent years, these tend to be increasingly replaced with solid forms. First, they can be easily processed in applications to electrical or electronical materials. Second, they can contribute to achievement of downsizing to provide lighter and more compact products and power saving.
Conventionally known proton conductive materials include both inorganic and organic substances. Examples of the inorganic substances include uranyl phosphate in the form of a hydrate. However, with respect to the inorganic compounds, there are many problems in forming the conductive layer on the electrode or substrate, because sufficient contact cannot be attained at the interface.
On the other hand, examples of the organic substances include polymers involved in so-called cation exchange resins, such as e.g., sulfonated vinyl polymers such as polystyrene sulfonic acid, perfluoroalkyl sulfonic acid polymers such as NAFION® (product name, by DuPont), perfluoroalkyl carboxylic acid polymers, and organic polymers of such heat resistant polymers as polybenzimidazole and polyetheretherketone having sulfonic or phosphoric group introduced therein.
Typically, in fuel cell production, an electrolyte consisting of the perfluoroalkyl sulfonic acid polymer is placed between the two electrodes, and then is subjected to heat processing such as hot pressing to obtain a membrane-electrode assembly. The heat distortion temperature of the fluorine-based membrane described above is comparatively low at around 80° C. so that junction processing can be readily executed. However, the temperature of the fuel cell may be raised to not lower than 80° C. due to the reaction heat generated in electrical power generation, and a creeping phenomenon occurs through softening of the electrolyte membrane, whereby both electrodes may short, resulting in a problem of failure in power generation.
To avoid such a problem, the fuel cell has been currently designed so as to increase the thickness of the electrolyte membrane to some extent, or to keep the operational temperature at not higher than 80° C. However, these solutions may lead to limitation of the highest output power generation. To improve the low heat distortion temperature and inferior mechanical properties at high temperatures of the perfluoroalkyl sulfonic acid polymer, a solid polymer electrolyte membrane using an aromatic polymer that is used as an engineering plastic has been developed.
For example, in U.S. Pat. No. 5,403,675, a solid polymer electrolyte membrane constituted with a rigid sulfonated polyphenylene is disclosed. This polymer is based on a polymer produced by polymerizing an aromatic compound having a phenylene chain and introducing a sulfonic acid group through a reaction with a sulfonating agent. The electrolyte membrane constituted with this polymer has superior creeping resistance at high temperatures, with a heat distortion temperature of not lower than 180° C.; however, extremely high temperature is required when such an electrolyte membrane is subjected to junction with the electrodes by way of hot pressing. In addition, there are problems of causing an elimination reaction or crosslinking reaction of the sulfonic acid group, and deterioration of the electrode layer when the electrolyte membrane is heated for extended periods of time at a high temperature.
In the meantime, in a fuel cell produced using a polymer electrolyte and a polymer electrolyte membrane in an electrode layer, it would be an important factor for enhancing power generation performance to allow a cation generated at the cathode to be efficiently and quickly conducted from the polymer electrolyte to the electrolyte membrane, and further to the anode via the polymer electrode membrane. Therefore, since a polymer electrolyte with superior cation conductivity is preferred, the content of protonic acid groups typified by a sulfonic acid group in the polymer electrolyte is preferably as high as possible.
In addition, unless the polymer electrolyte and the electrolyte membrane are used constantly under humid conditions during power generation, the performance may be deteriorated due to the reduction in cation conductivity, and occurrence of polarization. Therefore, many attempts have been made to increase the content of the protonic acid groups in the polymer electrolyte such that sufficient water retentivity is provided, see, for example, Japanese Unexamined Patent Application Publication Nos. 2004-51685, 2005-63778, 2005-139318 and 2005-113051. By thus increasing the water retentivity to indirectly maintain the humid condition, improvement in critical current density, simplification of the humidifier, and improvement of power generation performance can be expected.
However, in cases in which the content of protonic acid groups in the polymer electrolyte is excessively increased, when the polymer electrolyte and the electrolyte membrane come in contact with hot water generated during the solid polymer electrolyte fuel cell power generation, dimensional deformation may be increased by swelling and dissolution may occur. Thus, in a low temperature environment, the electrodes may be detached due to shrinkage of the electrolyte membrane, and the preferable power generation performance may not be achieved. In addition, when the electrolyte membrane is dissolved to form a pinhole, both electrodes may short, so that a phenomenon of failure in power generation may occur. Thus, the content of protonic acid groups in the polymer electrolyte for use in fuel cell is limited, thereby leading to restricted power generation performance.
Accordingly, an object of the present invention is to provide a membrane-electrode assembly for a solid polymer electrolyte fuel cell that exhibits superior dimensional stability to the high temperature of hot water generated on power generation, and that has both excellent power generation performance and durability in a low current environment and a low temperature environment.